1. Technical Field
This disclosure relates to a capacitor and a method of forming the same, and more particularly, to a capacitor having a dielectric layer that includes a material of high dielectric constant and a method of forming the same.
2. Description of the Related Art
As the memory cell area of a semiconductor device decreases, the capacitance of a capacitor in the cell decreases as well. A decreased data storage capacity of the memory cell results from the reduced capacitance. Accordingly, a refresh characteristic of the memory cell deteriorates and may require a high operation voltage.
For the above reasons, ways of improving the capacitance of a memory cell without decreasing a surface of the cell area of the semiconductor device have been explored. For example, increasing the effective surface area of the capacitor and/or the dielectric constant of the dielectric layer has been suggested as ways to improve the capacitance of the capacitor.
To increase the effective surface of a capacitor, a dielectric layer of a capacitor is formed as a thin layer because the cell area of a memory device is limited and defined, or a lower electrode of a capacitor may be formed as a vertical structure, for example, in a cylindrical shape. However, the thin dielectric layer may cause an increased current leakage from the capacitor, and the vertical structure of the lower electrode increases the manufacturing costs and reduces manufacturing efficiency due to various additional complicated processes that are required to produce the vertical structure.
Recently, dielectric layers of a capacitor have included a material having a dielectric constant (k) that is greater than that of silicon oxide (hereinafter referred to as a high-k material) in order to overcome the problems associated with increasing the effective surface of a capacitor. Examples of high-k materials include metal oxides such as aluminum oxide (Al2O3), tantalum oxide (Ta2O5), and hafnium oxide (HfO2).
The use of metal oxides as high-k materials is problematic because gases around the metal oxide easily react with the metal oxide when a heat budget is applied in a subsequent process, which rapidly decreases the dielectric constant. For this reason, a polysilicon layer that is used as an upper electrode of the capacitor cannot be formed directly on the metal oxide layer because the polysilicon layer is typically formed at a temperature greater than about 700° C.
To reduce this problem, a titanium nitride layer is typically formed on the metal oxide layer before the polysilicon layer is formed, thereby preventing a chemical reaction between the polysilicon layer and the underlying metal oxide layer. However, the process of forming the titanium nitride layer on the metal oxide layer may cause additional problems.
For example, when a titanium nitride layer is formed on a dielectric layer composed of a high-k material such as hafnium oxide, gases such as titanium chloride (TiCl4) gas and ammonia (NH3) gas are generally used as the processing gases. A diffusion rate for the titanium chloride gas is greater than that of the ammonia gas, so the titanium chloride gas remains on a bottom portion of a cylindrical capacitor. The titanium chloride gas reacts with the hafnium oxide at the bottom portion of the cylindrical capacitor, thereby producing hafnium chloride (HfCl4) on the bottom portion of the capacitor. The boiling point of the hafnium chloride is very low and it easily evaporates during the formation process for the capacitor, so a thickness of the hafnium oxide layer will be locally reduced. A thin dielectric layer causes a discharge of electrons stored in the capacitor, so that data stored in a cell is unfortunately erased.
Furthermore, the surface of the titanium nitride layer has a tendency to form cracks because the internal stress of the titanium nitride layer is very high due to deposition characteristics. In particular, when a lower electrode is formed as a hemispherical grained layer or a mesh-shaped supporting pattern, the cracking is more easily generated on the titanium nitride layer because the lower electrode has a wavy surface. Decreasing a thickness of the titanium nitride layer may prevent the generation of the cracking. However, when the thickness of the titanium nitride layer is too small, for example, below about 100 Å, the titanium nitride layer no longer functions as an effective barrier metal layer. The cracking of the titanium nitride layer causes current leakage, thereby degrading the operational characteristics of the capacitor.
Embodiments of the invention address these and other disadvantages of the conventional art.